Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] qdict: add qdict_steal()

[Marc-André Lureau]

Hi
On Fri, Aug 24, 2018 at 1:30 PM Markus Armbruster <address@hidden> wrote:
>
> Marc-André Lureau <address@hidden> writes:
>
> > Hi
> >
> > On Fri, Aug 24, 2018 at 10:05 AM Markus Armbruster <address@hidden> wrote:
> >>
> >> Marc-André Lureau <address@hidden> writes:
> >>
> >> > Add a new function qdict_steal(), that deletes a key from a dict, and
> >> > returns the associated value, if any. Simplify related code.
> >> >
> >> > Signed-off-by: Marc-André Lureau <address@hidden>
> >> > ---
> >> >  include/qapi/qmp/qdict.h |  1 +
> >> >  monitor.c                |  3 +--
> >> >  qobject/block-qdict.c    |  7 ++-----
> >> >  qobject/qdict.c          | 22 ++++++++++++++++++++++
> >> >  4 files changed, 26 insertions(+), 7 deletions(-)
> >>
> >> Missing: tests/check-qdict.c update.
> >
> > ok
> >
> >>
> >> > diff --git a/include/qapi/qmp/qdict.h b/include/qapi/qmp/qdict.h
> >> > index 7f3ec10a10..84644bfba6 100644
> >> > --- a/include/qapi/qmp/qdict.h
> >> > +++ b/include/qapi/qmp/qdict.h
> >> > @@ -37,6 +37,7 @@ QObject *qdict_entry_value(const QDictEntry *entry);
> >> >  size_t qdict_size(const QDict *qdict);
> >> >  void qdict_put_obj(QDict *qdict, const char *key, QObject *value);
> >> >  void qdict_del(QDict *qdict, const char *key);
> >> > +QObject *qdict_steal(QDict *qdict, const char *key);
> >> >  int qdict_haskey(const QDict *qdict, const char *key);
> >> >  QObject *qdict_get(const QDict *qdict, const char *key);
> >> >  bool qdict_is_equal(const QObject *x, const QObject *y);
> >> > diff --git a/monitor.c b/monitor.c
> >> > index a1999e396c..eab2dc7b7b 100644
> >> > --- a/monitor.c
> >> > +++ b/monitor.c
> >> > @@ -4262,8 +4262,7 @@ static void handle_qmp_command(JSONMessageParser 
> >> > *parser, GQueue *tokens)
> >> >
> >> >      qdict = qobject_to(QDict, req);
> >> >      if (qdict) {
> >> > -        id = qobject_ref(qdict_get(qdict, "id"));
> >> > -        qdict_del(qdict, "id");
> >> > +        id = qdict_steal(qdict, "id");
> >> >      } /* else will fail qmp_dispatch() */
> >> >
> >> >      if (req && 
> >> > trace_event_get_state_backends(TRACE_HANDLE_QMP_COMMAND)) {
> >> > diff --git a/qobject/block-qdict.c b/qobject/block-qdict.c
> >> > index 42054cc274..a07664ee6a 100644
> >> > --- a/qobject/block-qdict.c
> >> > +++ b/qobject/block-qdict.c
> >> > @@ -692,8 +692,6 @@ void qdict_join(QDict *dest, QDict *src, bool 
> >> > overwrite)
> >> >   */
> >> >  bool qdict_rename_keys(QDict *qdict, const QDictRenames *renames, Error 
> >> > **errp)
> >> >  {
> >> > -    QObject *qobj;
> >> > -
> >> >      while (renames->from) {
> >> >          if (qdict_haskey(qdict, renames->from)) {
> >> >              if (qdict_haskey(qdict, renames->to)) {
> >> > @@ -702,9 +700,8 @@ bool qdict_rename_keys(QDict *qdict, const 
> >> > QDictRenames *renames, Error **errp)
> >> >                  return false;
> >> >              }
> >> >
> >> > -            qobj = qdict_get(qdict, renames->from);
> >> > -            qdict_put_obj(qdict, renames->to, qobject_ref(qobj));
> >> > -            qdict_del(qdict, renames->from);
> >> > +            qdict_put_obj(qdict, renames->to,
> >> > +                          qdict_steal(qdict, renames->from));
> >> >          }
> >> >
> >> >          renames++;
> >>
> >> How did you find possible uses for qdict_steal()?
> >
> > manual review
>
> Okay.  I expected a few more than two.
>
> >> > diff --git a/qobject/qdict.c b/qobject/qdict.c
> >> > index 3d8c2f7bbc..322f23f094 100644
> >> > --- a/qobject/qdict.c
> >> > +++ b/qobject/qdict.c
> >> > @@ -406,6 +406,28 @@ void qdict_del(QDict *qdict, const char *key)
> >> >      }
> >> >  }
> >> >
> >> > +/**
> >> > + * qdict_steal(): Steal a 'key:value' pair from the dictionary
> >>
> >> "Steal" is cute, but perhaps qdict_get_and_del() would be clearer.
> >
> > "steal" is common in glib. I don't mind renaming.
>
> Hmm.  I found g_hash_table_steal(), but it's a bit different: it
> "Removes a key and its associated value from a GHashTable without
> calling the key and value destroy functions" (that part seems close
> enough), but returns a bool rather than the value.  Feels like sub-par
> design to me.  Is this the precedence you have in mind?

They are many more _steal* functions in the GNOME libraries (search
with devhelp): the general idea is to remove an object from the owner
(without destrying), and return that object/reference.

-- 
Marc-André Lureau